Preparation of graphite yarns

ABSTRACT

A method of making graphite fibers with high-tensile strength, high modulus of elasticity and, in many instances, fibers of improved graphitic character is described. The method of producing such fibers, generally in the form of yarn, comprises rapidly bringing a stabilized synthetic polymer yarn to a temperature within the range of 1800*-3200* C. Stabilization of the synthetic polymer yarn, when necessary, is usually effected by heating the yearn at a selected temperature in the range of about 200*-500* C. in an oxidizing atmosphere. Among the preferred polymers are the acrylonitrile polymers and polymers of nitrogen-containing polycylic organic compounds.

Unites Ezekiel States Patent 51 Jan. m, 1972 [54] PREPARATIUN 0F GRAPlllll'lllE YARNS [72] Inventor: Herbert M. Ezekiel, Dayton, Ohio [22] Filed: May 28,1968

[21] Appl.N0.: 732,555

[52] U.S. Cl ..23/209.11, 264/29 [51] Int. Cl. ..C01lb31/07 [58] Field of Search ..23/209. 1; 8/1 15.5; 260/47 [56] References Cited UNITED STATES PATENTS 3,094,511 6/1963 Hill et all ..260/78 3,285,696 11/1966 Tsunoda ..23/209.l 3,399,252 8/1968 Hough et a1 t ..23/209.3 X 3,412,062 11/1968 Johnson et al.. ..23/209.1 X 3,449,077 6/1969 Stuetz ..23/209.1 3,528,774 9/1970 Ezekiel et al. .....23/209.1 3,540,848 11/1970 Krugler et a1 ..23/209.3

3,547,584 12/1970 Santangelo ..23/209.l

OTHER PUBLICATIONS Van Deusen, J. Polymer Science," Pt. 8, Vol. 4, 1966, pp. 211- 214.

Primary Examiner-Edward J. Meros Att0rney-Harry A. Herbert and Alvin E. Peterson [57] ABSTRACT A method of making graphite fibers with high-tensile strength, high modulus of elasticity and, in many instances, fibers of improved graphitic character is described. The method of producing such fibers, generally in the form of yarn, comprises rapidly bringing a stabilized synthetic polymer yarn to a temperature within the range of l8003200 C. Stabilization of the synthetic polymer yarn, when necessary, is usually effected by heating the yearn at a selected temperature in the range of about 200-500 C. in an oxidizing atmosphere. Among the preferred polymers are the acrylonitrile polymers and polymers of nitrogen-containing polycylic organic com pounds.

13 Claims, N0 Drawings PREPARATION OF GRAPHITE YARNS The invention described herein may be manufactured and used by or for the United States Government for govemmenta] purposes without payment to me of any royalty thereon.

BACKGROUND OF THE INVENTION The present invention deals with the formation of graphite yarns by the graphitization of fibers of polymeric materials. More particularly, the invention deals with the preparation of multiple-filament yarns with a high tensile strength, a high modulus of elasticity and, in selected instances, comprising carbon of improved graphitic character.

There has been an increasing demand for materials of construction of high strength-to-weight ratio and high modulusto-weight ratio for use in aerospace vehicles and devices par ticularly for materials which, additionally, have good thermal stability. Specifically there has been a demand for improved reinforcing fibers to be embodied in structural composites which form the components of the leading edges of high-speed aircraft, the nose cones or heat shields for reentry vehicles, rocket engine components, and the like.

Graphite and other carbon forms have already been used as reinforcing agents in such structures and, in particular, have been used in the form of graphite strands or fibers in the rein forcement of composite materials for aerospace structures. Such fibers have been made in a number of ways. An early type of fiber comprised whiskers resulting from the partial oxidation of natural gas. Another early type of carbon fiber was made by the carbonization of strands of cellulosic material such as strands of cotton. As the plastic field developed, many of the new synthetic polymers were substituted for cotton with advantage. Simple carbonization of such fibers, however, led to the production of very weak fibers with practically no resistance to mechanical stresses. Research was then directed to the improvement of the physical properties of the fibers.

An almost invariable practice in the art has been a multiple step, particularly three-step, treatment of synthetic polymer yarns. The three-step treatment can be defined as: first, a stabilization step which can include oxidation, cross-linking, cyclization, and other well-known polymer reactions and degradations for the temperature range involved; second, a carbonization step which converts the altered high molecular weight polymer fiber to a fiber consistingof a highly carbonaceous residue with as much as several percent each of various elements such as hydrogen, oxygen, nitrogen, and phosphorus depending upon the original polymer nature, the time and temperatures involved in the carbonization process, and the mode of stabilization proceeding carbonization; and, third, a graphitization step. Graphitization is referred to by the practitioners of this art in the sense of a high-temperature treatment, usually above 2,000" C. It is commonly recognized that this treatment has the effect of yielding materials of almost pure carbon but that the product may or may not be truly graphitic in the crystallographic sense and that the character of the precursor polymer has a pronounced bearing on the degree of graphitic character that will result from the treatment. Thus, certain polymers, such as polyvinyl chloride, are called graphitizing" and others, such as polyvinylidene chloride and polyacrylonitrile, are called nongraphitizing."

The term stabilized is used herein to describe fibers which have been treated so as 'to be particularly suitable for graphitization without rupturing or forming large voids or hollow structures during the-process. Such stabilized fibers are normally characterized by their increased oxygen content, their uniformity of color in cross sections thereof, and by the integrity of the fibers at all processing stages (that is, they dont stick together).

In addition to the division of the graphitization process into these three general steps or temperature ranges, the first two steps usually incorporate a gradual heating of the fibers by either utilizing slowly increasing temperatures and sometimes holding the fiber at the maximum temperature for that step or utilizing a series of temperatures within the particular step and holding the fibers for various times at each of these temperatures. Thus, French Pat. No. 1,430,803 (Jan. 24, 1966) teaches that polyacrylonitrile fibers are oxidized in the first of a three-step process by heating the fiber at about 220 C. for 24 hours in an oxidizing atmosphere, while holding the yarn under tension. Such low-temperature pretreatment of the fiber in an oxidizing atmosphere is designed to prepare, or stabilize, the fiber for subsequent carbonization although the patent does not specifically state such a purpose. The patent also describes, as the second step, carbonization processes for these fibers which require 24 to 66 hours for the gradual heating to the maximum carbonization temperature of l,000 C. Similarly, US. Pat. No. 3,107,152 teaches that flexible fibrous graphite may be produced from cellulosic material by slowly heating the material to about 400 C. during 6 to 30 hours, followed by a heating schedule of about 5 hours to reach a temperature of 900 C. and a subsequent graphitization step.

As indicated above, the workers in the art have stabilized synthetic polymer fibers in a number of ways prior to a second step of carbonizing the fibers at some relatively intermediate temperature and a third step of graphitizing the carbonized fiber at some relatively high temperature.

However, the need for improvements in the art continue for high-strength, high-modulus reinforcing fibers for the preparation of structural composites.

OBJECTS It is, therefore, an object of this invention to prepare a graphite fiber having high tensile strength, high elastic modulus, and, in many instances, a fiber of improved graphitic character.

It is a further object to produce such fibers in the form of yarn which can be handled without undue breakage and which is amenable to incorporation into composites with other materials to make high strength-to-weight ratio and high modulus-to-weight ratio aerospace structures.

It is a still further object to produce such yarns at relatively low cost and by a relatively simple technique wherein, in all cases, the intermediate heating stage of the usual three-step process of the art is unnecessary and wherein, in cases where the synthetic polymer of commerce is already sufficiently stable, both the step of stabilizing the fiber and the intermediate heating stage are unnecessary.

It is a specified object to provide for the direct graphitization of a stabilized synthetic polymer fiiber without a pregraphitization carbonization step.

SUMMARY OF THE INVENTION I have now found that the foregoing and related objects can be attained in a method of making graphite yarn with a high tensile strength and a high modulus of elasticity wherein said method comprises the step of rapidly bringing a stabilized synthetic polymer fiber to a temperature within the range of 18003200 C. in an inert atmosphere. The fiber, or yarn, may be exposed to such high temperature for as little as from one-fourth to 2 minutes whereby the stabilized fibers are directly graphitized without a pregraphitization carbonization step.

By rapidly" it is meant that the fiber is exposed immediately, or within a very short time after initiation of the heating, to a temperature in the range of 1800-3200 C. so that within a period of about 5 minutes or less after the fiber has reached a temperature of about 500 C. it is heated to a temperature of at least 1,800 C. Preferably the period of passing from a temperature of about 500 C. up to above l,lB00 C. should be effected within 2 minutes and even within 0.25 or 0.5 minute. In this way the prolonged heating period previously applied for the intermediate carbonization step is avoided and improved results are obtained, as described herein, by the very quick approach to the graphitization temperature.

While the method works satisfactorily if the yarn is held under a minimum tension so as to be able to be passed through the graphitization apparatus without touching the heating element and yields fibers with high tensile strength and modulus properties, I prefer that the yarn be held under selected additional tension since this often results in increased tensile strength and/or modulus, decreased fiber diameter, and a more uniform graphite fiber size distribution.

In the majority of instances the synthetic polymer yarns as formed are not sufficiently stabilized for the facile carrying out of the invention, and I prefer to pretreat the yarn in an oxidizing atmosphere at a temperature in the range of 200 to 500 C., preferably 265290 C. for acrylonitrile fibers, depending on the polymer and the time of exposure.

With respect to the step of stabilization of the polymer fiber (and as described in my copending application filed the same date herewith) I have found that, in the specific case of the acrylic polymers, it is uniquely advantageous to stabilize the fiber in an oxidizing atmosphere, preferably in air, at a temperature of 265-290 C., preferably at 275 C., for at least 3 hours, either in a batch or continuous operation and preferably in a fashion which will hold the fiber at approximately its original length or cause it to stretch a few percent during stabilization. This latter can be accomplished by using a tension selected to suit the particular yarn bundle or, conveniently, by winding the yarn on a cylinder at a specific, uniform tension.

The temperature of 265-290 C. is fairly critical. While it is not intended that the scope of the invention be restricted thereby, I believe the superior stabilization results achieved at that temperature may be due to (l) the attainment of a relatively high temperature without effecting any significant exothermic decomposition reactions in the acrylic polymer (as occur at various temperatures above 200 C., depending on the precise acrylic polymer, the size of the polymer sample, and the heat dissipation characteristics of the sample package and the heating method); (2) the effecting of complete stabilization throughout the diameter of the fiber in a reasonable short time (in contrast to times required at about 220 C.), for example, in about 7 hours for yarns comprised of 3.75-denier filaments and in about 3-5 hours for yarns comprised of 1.2-2.1-denier filaments; (3) the attainment of a favorable degree of oxidation, or oxygen pickup (typically about 10-20 percent by weight) concurrent with the other reactions, especially cyclization and aromatization, which are believed to occur under these conditions; and (4) the attainment of a highly favorable balance between the degree of oxidation of the polymer and the amount of rigidization of the fiber structure (due to cross-linking or analogous processes) which, during the graphitization step of the invention, stabilizes the fiber against rupture, pore or void formation, and the like, while still imparting to it the strength necessary to allow a wide range of tensions to be used during graphitization.

I believe that this last item above allows imparting to the fiber a higher degree of orientation during the graphitization than is possible with samples stabilized at either lower temperatures (such as 220 C.) or at higher temperatures (such as 300 C.). It has been shown that a wide range of tensile properties and fiber densities can be achieved by the different tensioning weights and temperatures used with yarns stabilized in this fashion. In order to attain the most desirable results with acrylonitrile polymer fibers, therefore, such fibers should (1) be stabilized by my improved stabilization step at about 275 C., as outlined above and described more fully in my said copending application, and then (2) be rapidly brought to a high temperature by my improved method of graphitizing stabilized fibers as described and covered herein.

As indicated previously, however, and if one does not wish to avail himself of my improved method of stabilizing acrylic polymer fibers he should, at least, stabilize acrylic fibers by a prior art method before subjecting said fibers to the direct high-temperature graphitization step of my invention. For example, this invention has been used to produce graphite fibers with high tensile strength and high modulus from acrylonitrile polymers which have been stabilized under the several following conditions: 40 hours at 220 C., or 25 hours at 275 C., or 1 hour at 300 C.

The yarns which find use in the method of the invention are preferably acrylonitrile polymers, including homopolymers, copolymers, terpolymers, graft polymers, and the like containing at least 50 percent acrylonitrile, advantageously at least 65 percent and preferably at least percent. For example, I prefer acrylonitrile and methyl acrylate, the copolymer of 97 percent acrylonitrile and 3 percent vinyl acetate, and the like copolymers which include about 85 percent or more acrylonitrile units. Typical commercial polymers are sold, for example, under the trademarks Courtelle and Dralon T."

Typical comonomers that can be used are: styrene, alphamethyl styrene, vinyl toluene, vinyl xylene, vinyl naphthalene, vinyl diphenyl, vinyl methylnaphthalene, vinyl acetate, vinyl chloride, acrylamide, dimethylacrylamide, methacrylonitrile, methyl methacrylate, ethyl acrylate, vinylidene chloride, vinylidene cyanide, phenyl vinyl ether, vinyl methyl phthalate, vinyl methyl maleate, vinyl butyl succinate, ethylene, propylene, butylene, amylene, decylene, etc.

Polymers containing 50 percent or more acrylonitrile can be used for the purpose of this invention depending on the particular properties desired in the ultimate product. However, for maximum strength properties it is desirable to have at least 65 percent acrylonitrile and preferably at least 85 percent acrylonitrile in the starting polymer material.

Also preferred are nitrogen-containing polycyclic polymers included in the classes such as polybenzimidazoles, for example poly[2,2-(m-phenylene)-5,5-(dibenzimidazole)], polyoxadiazoles, for example poly[l,3/ l ,4-phenylene-2,5-

( 1,3 ,4-oxidiazole polythiadiazoles, for example poly[ 1,3/1 ,4-phenylene-2,5-( l,3,4-thiadiazole)],

poly( bisbenzimidazobenzophenanthrolene the aromatic polyamides, the aromatic polyimides, and the like.

Poly(bisbenzimidazobenzophenanthroline) is an example of a polymer yarn which, as normally formed, is already sufficiently stabilized.

Illustrations of various nitrogen-containing polycyclic polymers that can be used in the practice of this invention are reported in the literature as follows:

Poly( bisbenzimidazo-benzophenanthroline Poly[2,2-(m-phcnylene)-5,5-bibenzimidazole)]:

L OO X l L J (J. Polym. Sci., N0. 19, 49 (1967)) Polypyromellitimide of arylene (Ar) diamines including p-phenylene diaminc, 4,4-diarninodiphenyl sulfide, 4,4-diaminodiphcnyl methane, etc.:

(J. Polym. Sci., N0. 15!, 41 (1067)) Polyamidcs from arylcnc dizunincs and aromatic dicarboxylic acids, such as:

NIICO Also the polyamidu having three molat structures pui- 1mm unit:

Formula E J. Polym. Sci., No. 10. 30 (1987i MN N N .ltllJtl-Ql. F

J. Polym. $01., No. 19, so (1067) Polyl l ,3/1,4-phcnylono-2,5(1,3/1,4tl1iudiazole)] J. lolym. 801., No. 1&1, S (1067) The density of the finished graphite yarn can be varied con siderably by variation of the graphitization temperature and by variation of the tension on the fibers. In the case of acrylic polymers, for example, treatment of the stabilized yarn at relatively higher temperatures in the 1800-3200 C. range and at relatively higher tensions result in a thinner and more dense yarn. Alternately, the lower density graphite yarns from acrylic polymers are made at relatively lower temperatures and relatively lower tensions. Lowdensity yarns having high strength-to-weight and modulus-to-weight ratios by virtue of the low density are preferred for aerospace parts, submarine hulls, and reentry structures.

Suitable tensions on the stabilized yarns during the graphitization step are not usually critical and an increased fiber diameter, an increased number of strands, and an increased number of twists usually require an increased tension for comparable results. Tensions up to 250 grams are usually sufficient but some stabilized polymer yarns are not strong enough to withstand tensions even as low as 150 grams and it may be necessary to test a specific yarn before a suitable tension is selected.

In my method of graphitizing a stabilized synthetic polymer yarn at a temperature in the range of l800-3200 C. I obtain graphite yarns which have a modulus of elasticity as high or higher than can be obtained in any other method and I obtain easily the high tensile strengths which are obtained only occasionally and only with difficulty by prior art methods. A particular advantage of my invention is that I can eliminate, with advantage, at least one step of the usual three-step method. Further and as pointed out above, the method applied to acrylic polymer yarns can yield graphite yarns of improved graphitic character.

While the advantages outlined above for a one-step or direct graphitization are obvious and significant in terms of cost and fiber properties, there are certain additional advantages in this invention that can be appreciated by a worker in this field. For example, I find that stabilized polymer fibers are characteristically both flexible and strong. Therefore, these fibers can be handled easily and passed through graphitization apparatus by tying the stabilized fiber to a hightemperature-stable leader (such as a commercial carbon yarn). This technique works for initial startup, but, more importantly, also permits rethreading the apparatus in the case of yarn breakage even at the operating temperatures of about 2,800 C. Further, my experience with carbonization of polymer yarns indicates that polymers not only become brittle but that fibers pass through one or more strength minina during carbonization. Thus the processing difficulties attendant to these properties are avoided and a step which is one of the major sources of physical damage and flaw introduction to the fibers is omitted.

The practice of this invention is best: illustrated by the following examples. These examples are given merely by way of illustration and are not intended to limit the scope of the invention in any way nor the manner in which the invention can be practiced. Unless specifically indicated otherwise, parts and percentages are given as parts and percentages by weight. Throughout the specification, where reference is made to polymers and polymerization, it is intended that these terms embrace copolymers and copolymerization" unless otherwise indicated.

EXAMPLE I An acrylonitrile homopolymer yarn consisting of filaments (each 3.75 denier) with no twist is wound on an aluminum tube in a single layer at a tension provided by a 60- gram riding pulley from which a ZOO-gram weight is suspended. The tube is placed in a circulating-air oven for 7 hours at 275 C. A length of the resulting stabilized yarn is then passed through a vertical induction furnace susceptor in an argon atmosphere at 2,800 C. under a tension provided by suspending 50 grams from the yarn. The yarn is moved at A inch per minute and the hot zone of the: susceptor is about A inch long resulting in the yarn being exposed to the 2,800 C. temperature for about 1 minute. The single-filament tensile strength of the resulting fibers is about 265,000 lb. per square inch and the initial modulus is 61,000,000 lb. per square inch. X-ray diffraction studies show that the yarn has a high degree of graphitic character.

EXAMPLE II The foregoing example is repeated except that the hightemperature step is carried out under lOO-gram tension and the final temperature is 2,500 C. The resulting single-filament tensile strength is 277,000 lb. per square inch and the initial modulus is 52,000,000 lb. per square inch.

EXAMPLE III Example I is repeated except that the high-temperature step is carried out with the yarn under 50 grams tension and moving through a hot zone of about 3,000 C. at a rate of 2 inches per minute. The resulting tensile strength of a single filament is 151,000 lb. per square inch and the initial modulus is 40,000,000 lb. per square inch.

EXAMPLE IV A yarn, comprising a copolymer of 94 percent acrylonitrile and 6 percent methyl acrylate and comprising 750 filaments (each 1.5 denier) with no twist, is wound on an aluminum tube in a single layer at a tension provided by a GO-gram pulley from which a 1,200-gram weight is suspended and placed in a circulating-air oven for 7 hours at 275 C. Portions of the yarn are then treated as indicated in table I:

TABLE I Temp. Rate Tension Tensile Strength Modulus C. inJmin. g. IbJsq. in. lb./sq. in.

Portions of the stabilized yarn are also passed through a furnace about 22 inches long at relatively more rapid rates.

A 6-ply polybenzimidazole yarn (0.4 turn per inch twist) is passed continuously through a circulating-air oven at a temperature of 455 C. and under a tension of 157 grams. The rate of movement of the yarn is such as to permit a 7-minute exposure of the yarn to the 455 C. temperature. During the stabilization step the yarn stretches about percent. The stabilized yarn under a tension of 50 grams is then passed at a rate of 2 inches per minute through a hot zone of about 50 inch at a temperature of 2,885 C. The single-filament tensile strength of the resulting fiber is 135,000 p.s.i. and the modulus of elasticity is 31,000,000 p.s.i.

EXAMPLE VI A 6-ply polybenzimidazole yarn (2 turns per inch twist) is batch oxidized at 455 C. for 15 minutes while wound on an aluminum roll. The yarn, although not as completely stabilized as the yarn of example V, is satisfactorily stabilized for most purposes. The resulting yarn is then passed, at a rate of 2 inches per minute and under a tension of 50 grams, through a hot zone of about 10 inch at 2,650 C. The single-filament tensile strength of the resulting fiber is 97,000 psi. and the modulus of elasticity is 26,000,000 p.s.i.

EXAMPLE VII An 800-denier yarn (384 filaments, 0.5 turn per inch twist) made from a copolymer of 99.5 percent acrylonitrile and 0.5 methyl acrylate is wound on an aluminum tube under a tension provided by a 60-gram riding pulley from which a 50- gram weight is suspended. The tube is placed in a circulatingair oven for 5 hours at 275 C. A length of the resulting stabilized yarn is then graphitized at 2,800 C. in the manner described in example I. A length of the stabilized yarn is similarly graphitized except at a temperature of 2,2 1 5 C. The resulting single-filament tests are shown in table 3.

EXAMPLE VIII A polyacrylonitrile homopolymer yarn consisting of 120 filaments (each 3.75 denier) with no twist is wound on a hightemperature glass tube (of approximately 96 percent silica content) at a tension provided by a -gram riding pulley from which a SO-gram weight is suspended. The tube is placed in a circulating-air oven for 7 hours at 275 C. A length of the resulting yarn is then graphitized at 2,520 C. and under a tension of l00 grams in the manner described in example I. The resulting single-filament tensile strength is 254,000 lb. per square inch and the initial modulus is 54,000,000 lb. per square inch.

EXAMPLE IX A yarn, comprising a copolymer of 94 percent acrylonitrile and 6 percent methyl acrylate and comprising 750 filaments (each 1.5 denier) with no twist, is wound on an aluminum tube in a single layer at a tension provided by a 60-gram pulley from which 1,000 grams is suspended and placed in a circulating-air oven for 24% hours at 275 C. Portions of the yarn are then treated as indicated in table IV.

polyacrylonitrile yarn is heated at 300 C. for 1 hour. Portions of the yarn are then treated as shown in table V.

TABLE V Temp. Rzfl'ension Tensile Strength Modulus C. in./min. g. lb./sq. in. lb.lsq. in.

EXAMPLE XI Three yarns, comprising those described in examples I, IV,

and VII (except that this last has a twist of 4-5 turns per inch) respectively, are each heated in single layers on aluminum tubes for 40 hours at 220 C. These stabilized yarns are then treated as indicated intable Vl:

TABLE VI Tensile Temp, Rate, Tension, strength, Modulus, Yarn listed C. in./min. gms. lbs./sq.in. lbs./sq.in.

Example I 2, 500 2.00 2 141, 000 28,000,000 Example IV g, Ego 2.00 2 244, 000 32, 000, 000

T ,6 0 0.5 5 123,000 37,000,000 Example I H {2, 850 0.5 5 87, 000 23,000,000

TABLE III EXAMPLE XII The procedure of example I is repeated a number of times Graphitization Tension Tensile Strength Modulus using individually in place of the polymer of that example the Temp. c. 1 i lbJsq, following copolymers respectwely:

Acrylonitrile-styrene -20 2.800 100 334,000 76,000,000 75 Acrylonitrile-vinyl chloride -15 Acrylonitrile-vinylidene chloride 90-10 Acrylonitrile-acrylamide 75-25 Acrylonitrile-methacrylonitrile 50-50 Acrylonitrile-styrene-acrylamide 80-10-10 Acrylonitrile-methyl methacrylate 60-40 Acrylonitrile-vinyl chloride-vinylidene chloride 50-25-25 In each case improvements in the tensile strength and modulus are noted as compared to the same copolymer stabilized according to prior art method of example VIII.

EXAMPLE XIII The procedure of example I is repeated a number of times using individually in place of the polymer of that example the respective polycyclic polymers of formulas A trough G. In each case improvements in the tensile strength and modulus are noted.

It is to be understood that while specific examples describe preferred embodiments of my invention, they are for the purpose of illustration only, that the products and methods of the invention are not limited to the precise details and conditions disclosed, and that various changes may be made therein without departing from the spirit of the invention which is defined by the following claims:

Iclaim:

l. A process for graphitizing polymeric fibers to form high tensile strength, high modulus of elasticity graphite fibers, comprising the steps of stabilizing synthetic polymer fibers by heating said fibers in an oxidizing atmosphere at a temperature in the range of about 200-5 C., and thereafter rapidly heating said stabilized synthetic polymer fibers in an inert atmosphere to a temperature in the range of 1,800-3,200 C., the heating of said fibers being such as to effect an increase in temperature from 500 C. up to a temperature of at least l,800 C. in no more than about minutes, said synthetic polymer being selected from the group consisting of acrylonitrile polymers, polybenzimidazoles, polyoxadiazoles, polythiadiazoles, poly(bisbenzimidazobenzophenanthroline), and aromatic polyimides.

2. The process of claim I in which said polymer is an acrylonitrile polymer having at least 50 percent by weight of acrylonitrile in the polymer molecules thereof.

3. The method according to claim ll wherein the said synthetic polymer fibers are comprised of acrylonitrile polymers, and the acrylonitrile polymers are stabilized by heating in the range of 265-290 C., for at least 3 hours.

4. The method of claim 3 wherein said heating is at about 275 C. for 3-7 hours.

5. The method according to claim 1 wherein said polymer fibers are in the form of a yarn and are under tension.

6. The method according to claim 1 wherein said graphitization heating is carried out for about 0.5-2 minutes.

7. The method according to claim ll wherein said polymer fibers are in the form of yarn, said yarn is under tension, and said graphitization heating is carried out for about 0.25-2 minutes.

8. The method according to claim I wherein said polymer is an acrylonitrile copolymer containing at least percent by weight of acrylonitrile in the polymer molecule.

9. The method according to claim I wherein said polymer is an acrylonitrile homopolymer.

10. The method according to claim I wherein said polymer is a copolymer of approximately 97 percent by weight acrylonitrile and 3 percent by weight of'vinyl acetate.

11. The method according to claim I wherein said polymer is an acrylonitrile copolymer containing at least 65 percent by weight acrylonitrile.

12. The method according to claim I wherein said polymer is poly[2,2-(m-phenylene )-5,5-(dibenzimidazole)].

13. The method according to claim 1 wherein said polymer is poly(bisbenzimidazobenzophenanthroline). 

2. The process of claim 1 in which said polymer is an acrylonitrile polymer having at least 50 percent by weight of acrylonitrile in the polymer molecUles thereof.
 3. The method according to claim 1 wherein the said synthetic polymer fibers are comprised of acrylonitrile polymers, and the acrylonitrile polymers are stabilized by heating in the range of 265*-290* C., for at least 3 hours.
 4. The method of claim 3 wherein said heating is at about 275* C. for 3-7 hours.
 5. The method according to claim 1 wherein said polymer fibers are in the form of a yarn and are under tension.
 6. The method according to claim 1 wherein said graphitization heating is carried out for about 0.5-2 minutes.
 7. The method according to claim 1 wherein said polymer fibers are in the form of yarn, said yarn is under tension, and said graphitization heating is carried out for about 0.25-2 minutes.
 8. The method according to claim 1 wherein said polymer is an acrylonitrile copolymer containing at least 85 percent by weight of acrylonitrile in the polymer molecule.
 9. The method according to claim 1 wherein said polymer is an acrylonitrile homopolymer.
 10. The method according to claim 1 wherein said polymer is a copolymer of approximately 97 percent by weight acrylonitrile and 3 percent by weight of vinyl acetate.
 11. The method according to claim 1 wherein said polymer is an acrylonitrile copolymer containing at least 65 percent by weight acrylonitrile.
 12. The method according to claim 1 wherein said polymer is poly(2,2''-(m-phenylene)-5,5''-(dibenzimidazole)).
 13. The method according to claim 1 wherein said polymer is poly(bisbenzimidazobenzophenanthroline). 